1. Field Of The Invention
The present invention relates generally to digital data recording. More particularly, the invention relates to the encoding of an address mark which is used to achieve synchronization within a stream of data bits.
2. Description Relative To The Prior Art
In the recording of digital data on a record storage medium--like magnetic tape, a disk (magnetic, magneto-optical, optical) or an equivalent thereof--the bits recorded are systematically arranged for data retrieval. With a disk, for example, a bit stream is divided into a plurality of sectors, each of which contains a predetermined number of bits, which are commonly arranged into two groups. The first group of bits is known as a sector header or preamble and the second group, which follows the sector header, is known as a data field.
Note that the use of the term "divided" is somewhat misleading since there may or may not be actual physical separation between the two groups of bits. Rather, the bits are recorded and/or played back as a continuous, uninterrupted bit stream, while the terms sector header and data field merely serve as a convenient means of identifying a particular portion of the bit stream.
The bits in a sector header include primarily record-keeping information for facilitating data transfer. A disk sector header, for example, may include variable frequency oscillator (VFO) synchronization bits, phase synchronization bits, a sector address, etc.
A data field primarily includes user data bits, error detection and correction bits, as well as servo information for tracking control. The data bits, in turn, may further be divided into minisectors containing a specific number of bits.
Either the sector header or the data field, or both, commonly also includes bits corresponding to one or more address marks. An address mark serves advantageously to achieve byte (a set of eight binary bits) synchronization--identifying the beginning of a sector header or data field. Because of its importance, multiple address marks within a given sector serve for redundantly providing byte synchronization.
In order to transmit binary bits via a transmission channel or to record them on a disk or magnetic tape, the bit stream is usually modified--an operation known as channel coding. A known manner of channel encoding is the so-called Miller technique, known also as delay modulation mark (DMM) or modified frequency modulation (MFM).
MFM and DMM are functionally identical. The difference between the two schemes is that MFM uses isolated pulses for representing an encoded bit stream, whereas DMM uses the MFM pulse train to produce a 2-state signal, a state transition occurring at every MFM pulse. MFM is most suitable for magnetic recording, whereas DMM is preferred for optical recording. An advantage of coding of either type is that the data handling capacity of a transmission channel is effectively increased because the encoded bit stream is self-clocking, thereby obviating the need to transmit and to record a synchronous timing signal.
For magnetic recording, the Miller technique is defined by the following well known encoding rules:
1. A binary 1 bit is encoded as an MFM pulse at the center of the corresponding bit cell. This pulse is commonly called a data bit.
2. A binary 0 bit is encoded as an MFM pulse at the boundary between its bit cell and the immediately following bit cell so long as the latter cell also contains a binary 0 bit. This MFM pulse is commonly called a clock bit. For optical recording--DMM--a signal transition, high to low or low to high, occurs in synchronism with each MFM pulse.
FIG. 1 is useful in explaining the Miller technique; FIGS. 1(b) and 1(a) show, respectively, an arbitrary sequence of binary bits and their corresponding bit cells, denoted T; FIG. 1(c) shows the bit sequence of FIG. 1(b) in non-return-to-zero (NRZ) form; FIGS. 1(d) and 1(e) show, respectively, the corresponding sequence of MFM pulses--clock and data bits--and DMM signal transitions. Note from FIGS. 1(d) and 1(e) that the time between successive signal transitions, either MFM pulses or DMM transitions, is at least one bit cell, T, but is never greater than two bit cells, 2 T. A "1-T spacing" occurs whenever there are two consecutive binary 1 bits or three consecutive binary 0 bits; a "1.5 T spacing" occurs whenever a 00 binary pattern precedes or follows a binary 1 bit; a "2-T spacing", on the other hand, is produced only by a 101 binary pattern.
A well known problem that must be handled in digital data recording is the need to detect an address mark in the stream of binary bits. It is known in the prior art to encode bits, representing an address mark, using a unique signal, either MFM or DMM, so that the address mark will not be mistaken for other bits, such as data, in the bit stream. U.S. Pat. Nos. 3,750,121 and 4,319,287 disclose an address mark pattern that is different from any other pattern that results when a bit stream is encoded. It is intended, of course, that such an address mark pattern would not be confused with other coded bits--sector header or data--in the bit stream.
A coded address mark, disclosed in U.S. Pat. No. 3,750,121, consists of alternating DMM pulses of 1.5 T and 2 T duration. Such a pattern of pulses is not achievable under normal Miller encoding rules because a 2 T pulse always corresponds to a "101" pattern, and, therefore, must always begin and end at the middle of a bit cell.
Similarly, encoded address mark patterns, derivable from U.S. Pat. No. 4,319,287, are also different from other normally encoded patterns. This is because each address mark pattern is constrained to have two pairs of successive MFM pulses separated by two bit cells (2 T) one pair of which, like the DMM address mark of the '121 patent, is forced to begin and end at the boundary between adjacent bit cells.
Although each address mark of the aforementioned prior art is different from any signal pattern that results when a bit stream is encoded under normal Miller rules, the pattern for each address mark still must satisfy the "1 T min-2 T max" rule of Miller coding. With each address mark of the aforementioned patents being relatively close in appearance to a normally encoded bit stream, a decoding operation requires that each signal transition be resolved precisely to accurately distinguish an encoded address mark from encoded data patterns.